Inner diameter (mm) to US / Canada ring size
The short answer
By the formula, Inner diameter (mm) 18 is US / Canada 7.75.
Inner circumference 56.5 mm. Ring sizes are defined by that one measurement, so this conversion is exact up to rounding: US sizes are rounded to the nearest quarter, UK letters to the nearest half.
| Inner diameter (mm) | US / Canada |
|---|---|
| 14 | 3 |
| 14.5 | 3.5 |
| 15 | 4.25 |
| 15.5 | 4.75 |
| 16 | 5.5 |
| 16.5 | 6 |
| 17 | 6.75 |
| 17.5 | 7.25 |
| 18 | 7.75 |
| 18.5 | 8.5 |
| 19 | 9 |
| 19.5 | 9.75 |
| 20 | 10.25 |
| 20.5 | 11 |
| 21 | 11.5 |
| 21.5 | 12.25 |
| 22 | 12.75 |